Carbonyl clusters condensation reactions

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

Initial attack of hydroxide ions onto Ni(CO)4 gives as yet uncharacterized species,1 which readily condense with unreacted Ni(CO)4 to give variable mixtures of [Ni5(CO)12]2- and [Ni6(CO)12]2-. Hydrolysis of [Ni5(CO)12]2- in water produces [Ni6(CO)12]2-, according to the above reaction. The dianion [Ni CO) 2]2- is isolated as the tetramethylam-monium salt in a crystalline state in 70% yield. The [Ni6(CO)12]2 dianion is a starting material for the synthesis of several other homo- and heteromet-allic nickel carbonyl cluster complexes.1-9... 

Finally, cobalt carbide-carbonyl clusters have recently been isolated through a two-step synthesis. First of all, the well-known Co3(CO)9CCl is prepared from Co2(CO)8 and CCh, and then the hexanuclear carbide dianion [Co6(CO)i5C]2- is obtained in good yields (9) by further reaction with Xa[(, o(CO)4] in diisopropylether [see Eqs. (18) and (19)]. Further redox condensation between [Oo6(CO)i5C]2-and Co4(CO)i2 [see Eq. (11)] gives the square antiprismatic octanuclear cluster [Cdo8(Cdt )i8 C]2- (13). Both these carbide derivatives, as well as all of the other cobalt high nuclearity clusters, are sensitive to air and react with carbon monoxide at atmospheric pressure. 

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

A few homometallic HNCC have also been produced by the condensation reactions of anionic carbonyl clusters with cationic complexes. For instance, sequential buildup of rhodium HNCC via incorporation of Rh(CO)2 fragments has been achieved by reacting [Rh(CO)2(MeCN)2] in acetonitrile with a series of anionic rhodium clusters as illustrated by the reactions shown in Eqs. (11) and (12) (223). 

Few successful reactions are known by which an organometallic nitrosyl product has been formed from the condensation of mononuclear metal nitrosyl complexes. The unique cluster (Tj -C5H5)3Mn3(NO)4 (39), which contains the only i3-NO known (7) (40), is prepared using photochemical or thermal (41) decarbonylation of (Tj -C5H5)2Mn2(CO)2(NO)2 [Eq. (25)]. The use of the redox condensation of a nitrosyl carbonylmetallate with a neutral metal carbonyl cluster to produce a nitrosyl cluster has... 

The reaction of metal carbonyl complexes with carbonyl anions is a widely used technique for synthesis of mixed-metal clusters. This reaction is also termed reductive condensation or redox condensation. The metal carbonyl anion may be generated in situ or may be separately isolated. The mixed-metal product is usually an anion protonation can be used to generate a neutral cluster hydride. 

The general feature of syntheses of carbonyl clusters in most cases would appear to involve a condensation reaction of labile intermediates which have a lower nuclearity than that of the final product. One approach is via the production of coordinatively unsaturated species under conditions in which they can stabilize themselves only by condensation reactions. Coordinatively unsaturated species have been produced in a variety of ways, involving simple metal salts or carbonyl compounds as starting materials. 

Condensation reactions can also occur between compounds which are not coordinatively unsaturated. The condensation of metal carbonyls and carbonyl anions provide an example of this reaction. Such a reaction is that of iron carbonyl with the trinuclear carbonyl anion 192). Synthesis of carbonyl clusters often involves a predisproportionation step... 

Hieber and his school have carried out a systematic investigation of the transition metal carhonyl anions and derivative hydrides 169), and have synthesized many carbonyl anion and hydride clusters, particularly of the first row transition metals. The preparations of these clusters were all based on the reduction of a carbonyl compound. More recently a number of heteronuclear carbonyl anion and hydride clusters have been produced by condensation reactions. 

In many of the reactions classified as reduction of a metal carbonyl compound (Section II,C,1) it is implicit that a condensation reaction occurs, although the condensation may be followed by disproportionation. There are a number of syntheses of carbonyl metallate clusters in which this condensation is explicit. Fe(CO)5 reacts with Fe3(CO)f7 to form Fe4(CO)i3 as in Eq. (12) 186). This reaction has been used in particular to synthesize a number of heteronuclear clusters. 

Redox condensation can occur by the simple addition of a mononudear carbonyl cationic spedes to an anionic cluster as shown in equation (3.18). Such condensation reactions allow for a stepwise and controlled construction of higher nuclearity clusters whereby the regeneration of a suffidently high negative charge on the cluster can be accomplished by appropriate reduction or deprotonation steps in the condensation sequence. [168]... 

Some very large carbonyl cluster anions of the late transition elements have been prepared by condensation of carbonyl anions with polynuclear carbonyls. The first example of such a reaction was discovered by Hieber in 1965. 

Reaction of metallates with cationic complexes. The synthesis of one of the largest metal carbonyl cluster structurally characterized up to date, [Ni38Pt6(CO)4sH2] ", has been obtained from the condensation reaction of the metallate [Ni6(CO)i2] with the platinum(II) salt PtCl2. 

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